The goal of this research project is to define the characteristics of single nephron function during post-natal maturation by using the technique of micropuncture. Experiments are performed in guinea pigs of various ages in order to characterize the developmental changes in sodium transport at the level of the proximal and distal nephron. End proximal recollections and collections of final urine in hydropenic and saline loaded animals are used for this purpose. Stationary microperfusions of isosmotic raffinose are used to assess the tubular permeability to water, raffinose and sodium, as well as to determine the change in the limiting sodium concentration gradient with maturation. The permeability of the tubular membrane to macromolecules is tested in experiments in which substances of known molecular weight are injected into the proximal tubule, close to the glomerulus, and collected in the final urine. The ratio between each of these substances and inulin provides an estimate of intercellular channel permeability.